1. Field of the Invention
The present invention relates to a digital optical communication device for transmitting and receiving optical signals involving subcarriers.
2. Description of the Background Art
Digital optical communication has been recently utilized over a wide range of application. For example, infrared digital optical communication is widely applied to remote control for household electric products such as televisions, videos and the like.
Various methods have heretofore been contrived as systems of digital optical communication. Typical examples of these systems are amplitude shift keying (ASK) modulation, frequency shift keying (FSK) modulation, phase shift keying (PSK) modulation, and the like. When applied to digital optical communication, these modulation systems can be roughly classified into two types of communications, coherent optical communications and incoherent optical communication. The coherent communication is adapted to perform modulation by employing an optical medium itself as carriers, and the incoherent communication is adapted to perform modulation by carriers simulatively created by on-off controlling light in a cycle considerably slower than its wavelength. The carriers simulatively created in case of the incoherent communication are generally called subcarriers.
FIGS. 1A to 1C show pulse signals of the respective modulation systems. FIG. 1A shows the pulse signal of the ASK modulation system. When sections {circle around (1)} to {circle around (5)} divided by dotted lines are referred to as symbols, each symbol involves a plurality of pulses (subcarriers). The symbols {circle around (1)}, {circle around (3)} and {circle around (4)} involve subcarriers of the same frequency respectively, and indicate logic xe2x80x9c1xe2x80x9d. On the other hand, the symbols {circle around (2)} and {circle around (5)} involve no subcarriers respectively, and indicate logic xe2x80x9c0xe2x80x9d.
FIG. 1B shows the pulse signal of the FSK modulation system. The symbols {circle around (1)}, {circle around (3)} and {circle around (4)} involve subcarriers, and indicate logic xe2x80x9c1xe2x80x9d. The symbols {circle around (2)} and {circle around (5)} also involve subcarriers, which are different in frequency from those in the symbols {circle around (1)}, {circle around (3)} and {circle around (4)}. Due to the different frequency of the subcarriers, the symbols {circle around (2)} and {circle around (5)} indicate logic xe2x80x9c0xe2x80x9d.
FIG. 1C shows the pulse signal of the PSK modulation system. The symbols {circle around (1)}, {circle around (3)} and {circle around (4)} involve subcarriers, and indicate logic xe2x80x9c1xe2x80x9d. The symbols {circle around (2)} and {circle around (5)} also involve subcarriers, which are identical in frequency to but out of phase with those in the symbols {circle around (1)}, {circle around (3)} and {circle around (4)}. Due to the phase difference between the subcarriers, the symbols {circle around (2)} and {circle around (5)} indicate logic xe2x80x9c0xe2x80x9d.
The subcarriers, which are generally formed by simply controlling light on-off as described above, are substituted not as sine waves but as rectangular waves in general. As shown in FIG. 2, light emission and no emission are repeated in each symbol in a constant cycle for generating subcarriers in the ASK modulation system. An interval corresponding to one cycle of the subcarriers is hereinafter referred to as a slot.
In the aforementioned infrared remote control, a technique of modulating transmit data by a PPM (pulse position modulation) system and modulating certain carriers again by a waveform modulated in the PPM system is generally employed as one of many data transmission systems. FIGS. 3A and 3B illustrate an exemplary waveform of this data transmission system. While FIG. 3A illustrates a PPM modulated waveform in a broad view, each pulse consists of subcarriers, as shown in FIG. 3B. The PPM modulation system is adapted to transmit data by pulse positions, and pulse spaces Tp and Tp/2 indicate xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d respectively in FIG. 3A.
FIGS. 4A and 4B illustrate a conventional digital optical transmitter 721 and a conventional digital optical receiver 725 for the transmission system generally employed in the aforementioned infrared remote control, for example. The digital optical transmitter 721 includes a PPM modulation part 722 for receiving transmit data and PPM-modulating the same, an ASK modulation part 723 for ASK-modulating a PPM modulated signal, and an electrical/optical (E/O) conversion part 724 for converting an electrical modulated signal to an optical modulated signal. The PPM modulation part 722 receives the transmit data, for generating and outputting the aforementioned PPM modulated signal shown in FIG. 3A with no superposition of subcarriers. The ASK modulation part 723 modulates subcarriers by the PPM modulated signal inputted therein, and outputs the signal shown in FIG. 3B. The E/O conversion part 724 converts the electrical modulated signal received from the ASK modulation part 723 to an optical on-off signal and outputs the same.
The digital optical receiver 725 includes an O/E (optical/electrical) conversion part 726 for converting an optical modulated signal to an electrical modulated signal, an ASK demodulation part 727 for demodulating the electrical modulated signal from the O/E conversion part 726 in the ASK system, and a PPM demodulation part 728 for receiving the ASK demodulated signal and converting the same to receive data. The O/E conversion part 726 receives the optical modulated signal, and converts the optical on-off signal to an electrical modulated signal. The ASK demodulation part 727 outputs an ASK demodulated signal (PPM modulated signal) obtained by removing the subcarriers involved in the electrical modulated signal. The PPM demodulation part 728 converts the ASK modulated signal to receive data and outputs the same.
FIG. 5A is a circuit diagram of the O/E conversion part 726 and the ASK demodulation part 727, and FIGS. 5B to 5E illustrate output waveforms of the respective components. Referring to FIG. 5A, the O/E conversion part 726 includes a photoreceptor 731, which is an element converting received light to an electric current. The ASK demodulation part 727 includes an amplifier 732, a limiter 733, a bandpass filter (BPF) 734, a rectifier 735, an integrator 736, and a comparator 737. The amplifier 732 converts the current received from the photoreceptor 731 to a voltage and amplifies the same.
The limiter 733 suppresses a voltage exceeding a certain value. The BPF 734, which is adapted to remove noise components from subcarriers, matches its center frequency with the frequency of the subcarriers. When the photoreceptor 731 receives the optical signal shown in FIG. 5B, its output is converted to the signal shown in FIG. 5C through the amplifier 732, the limiter 733 and the BPF 734.
The rectifier 735 extracts only a plus component of the voltage. The integrator 736 integrates the output from the rectifier 735, and outputs the signal shown in FIG. 5D.
The comparator 737, which is formed by a Schmidt buffer, converts the output of the integrator 736 to a rectangular waveform as shown in FIG. 5E, and outputs the same.
While the digital optical transmitter and the digital optical receiver employing subcarriers have been described, an advantage of employment of the communication waveform using subcarriers is now described.
While the spectrum of a non-modulated signal (baseband signal) is generally distributed in a low-frequency region, this signal spectrum shifts to a band around the frequency of subcarriers when the subcarriers are modulated by a modulated signal. Particularly in the case of infrared communication, a number of external noises exist in the low-frequency region. Therefore, it is possible to improve the signal-to-noise ratio by modulating the modulated signal (baseband signal) by subcarriers and moving its signal spectrum to a high-frequency region having less noise. This is one of the preferable reasons why the communication waveform employing subcarriers is used in optical communications.
Description is now made with reference to a more concrete example.
Consider a transmission system employed in the aforementioned infrared remote control, for example. In this case, the modulated signal before modulation of the subcarriers is the PPM modulated signal shown in FIG. 3A, and the spectrum of this PPM modulated signal spreads in a low-frequency band of about 0 to 2 KHz. After modulation of subcarriers of 40 KHz, for example, the spectrum of the modulated signal (FIG. 3B) appears on either side of the frequency of the subcarriers. Thus, this spectrum appears in the range of 38 KHz to 42 KHz in the end. On the other hand, external infrared noise from by a general fluorescent lamp are distributed in the range of 50 to 60 Hz and harmonics thereof. While the frequency band of the spectrum of the original modulated signal (PPM modulated signal) itself overlaps with the same, it is understood that the spectrum of the signal after modulation of the subcarriers does not overlap with that of the infrared noise from by the fluorescent lamp and is hardly influenced by these noise sources.
On the other hand, a 500 KHz ASK system is also generally employed as a data transmission system for infrared data communication. FIGS. 6A and 6B illustrate an exemplary waveform of this data transmission system. The 500 KHz subcarriers are superposed on an unmodulated waveform, and each symbol involving the 500 KHz subcarriers indicates data xe2x80x9c0xe2x80x9d, and that involving no 500 KHz subcarriers indicates data xe2x80x9c1xe2x80x9d. Assuming that the transfer rate for main data is 19200 bps, the interval between the main data bits is 52.08 xcexcs and hence it follows that 26 slots exist per symbol.
FIG. 7 is a block diagram showing the structure of a conventional digital optical communication device. This digital optical communication device includes an ASK transmitter 811 including an ASK modulation part 813 for receiving a serial data signal and ASK-modulating the same and an E/O (electrical/optical) conversion part 812 for converting an electrical modulated signal to an optical modulated signal, an ASK receiver 814 including an O/E (optical/electrical) conversion part 815 for converting the optical modulated signal to an electrical modulated signal and an ASK demodulation part 816 for demodulating the electrical modulated signal from the O/E conversion part 815 in the ASK system, inverters 817 and 818, and an UART (universal asynchronous receiver and transmitter) 819 for performing serial-parallel conversion of data.
The UART 819 converts parallel data received from a computer (not shown) or the like to 8-bit non-parity serial data and outputs the same, as shown in FIG. 6A. The serial data involves a start bit (STA), 8-bit data, and a stop bit (STO). The ASK modulation part 813 modulates subcarriers by the inputted serial data, and outputs the signal shown in FIG. 6B. The E/O conversion part 812 converts an electrical modulated signal received from the ASK modulation part 813 to an optical on-off signal and outputs the same.
The O/E conversion part 815 receives an infrared receive signal (optical modulated signal), and converts the optical on-off signal to an electrical modulated signal. The ASK demodulation part 816 outputs an ASK demodulated signal obtained by removing the subcarriers involved in the electrical modulated signal. The UART 819 converts an inverted signal of the ASK demodulated signal and converts the same to parallel data. The UART 819 outputs the parallel data to the computer (not shown) or the like.
In order to increase the data communication channel capacity in a device utilizing the aforementioned digital optical transmission system, the transfer rate may be increased or the communication system may be changed. In this case, however, the device utilizing the conventional digital optical transmission system cannot make communication with other conventional devices such as a computer, for example.
An object of the present invention is to provide a digital optical transmitter/receiver having a larger data communication channel capacity than the conventional digital optical communication device device while maintaining compatibility with other conventional devices such as computers.
According to a certain aspect of the present invention, a digital optical transmitter for transmitting data through a modulation system utilizing subcarriers includes a main/subdata modulation part for distorting a main data modulated waveform obtained by modulating subcarriers in main data in response to subdata, and an E/O conversion part for converting an electrical modulated signal obtained as a result to an optical modulated signal and outputting the same.
A subcarrier modulated signal by the main data is distorted in response to the subdata, whereby communication is made with attachment of the data contents of the subdata to the main data. Consequently, the data communication channel capacity can be increased.
According to another aspect of the present invention, a digital optical receiver for receiving data through a modulation system utilizing subcarriers includes an O/E conversion part for receiving an optical modulated signal, converting the same to an electrical modulated signal and outputting the same, and a main/subdata demodulation part for detecting a distortion in subcarriers forming the electrical modulated signal and extracting subdata superposed in the subcarriers.
The main/subdata demodulation part detects the distortion in the subcarriers, thereby extracting the subdata. Thus, the data communication channel capacity can be increased.
According to still another aspect of the present invention, a digital optical transmitter for transmitting data through a modulation system utilizing subcarriers includes a main/subdata modulation part for distorting a non-emitting symbol of a main data modulated waveform obtained by modulating subcarriers on the main data in response to subdata, and an E/O conversion part for converting an electrical modulated signal obtained as a result to an optical modulated signal and outputting the same.
The non-emitting symbol of the main data modulated waveform is distorted in response to the subdata, whereby communication can be made with attachment of the data contents of the subdata to the main data. Consequently, the data communication channel capacity can be increased.
According to a further aspect of the present invention, a digital optical receiver for receiving data through a modulation system utilizing subcarriers include an O/E conversion part for receiving an optical modulated signal, converting the same to an electrical modulated signal and outputting the same, and a main/subdata demodulation part for detecting a distortion of a non-emitting symbol forming the electrical modulated signal and extracting subdata superposed in the non-emitting symbol.
The main/subdata demodulation part detects the distortion of the non-emitting symbol and extracts the subdata. Thus, the data communication channel capacity can be increased.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.